While I agree that learning assembler programming is an essencial part using embedded processors 1 I do think that insisting on only using assembler is akin to insisting on only building op-amps and logic gates from descreet components,it can be done but it isnt because its not very good design practice.
C is used because its 2 portable,easy to debug,modify,document,can be easily split over many developers,has many standard libraries etc,etc.
re 1) Very few still insist that pure assembler is the way to go, I think that today there are more pure C than pure assembler advocates. My personal opininon is "the right mix of the two".
re 2) I really think that the portability of C in the small embedded world is a myth. While calculations and such indeed will be portable, the predominance of I/O in these designs make a porting a huge task regardless of the language

Erik